With respect to a process for obtaining amino acids by enzymatically hydrolyzing a protein starting material containing a vegetable protein in a solid state, a large number of processes have been already known.
For example, JP-A-51-35461 describes a process for producing a liquid seasoning comprising, in combination, a first step of reacting modified de-fatted soybeans having a soluble nitrogen index of 50 or less with an alkaline protease having a pH of from 9 to 12 for 2 hours, thereby solubilizing and extracting 70% or more of a protein-derived nitrogen component to conduct solid-liquid separation, and a second step of subjecting the extracted to hydrolysis with a peptidase in a sealed container at from 40° C. to 60° C.
Further, JP-A-6-125734 describes an enzyme preparation which is obtained from an organic solvent-dipped product of koji formed through solid culture of a microorganism and which contains an exopeptidase obtained through autolysis of the koji, and a process for producing a proteinous liquid seasoning, which comprises reacting an animal or vegetable protein material with protein solubilizing enzymes, and then reacting the reaction product with a exopeptidase-containing enzyme preparation.
Still further, JP-A-9-75032 describes a process for producing a seasoning, wherein when koji for soy sauce is charged in the presence of alcohol for enzymatic hydrolysis at from 35° C. to 45° C., evaporation is forcibly conducted such that the alcohol concentration in the completion of the hydrolysis is 2% or less, and this hydrolyzate is fermented and aged.
Furthermore, JP-A-9-121807 describes a multi-purpose seasoning having a high glutaminate content and having a flavor peculiar to an acid-hydrolysis seasoning without a soy sauce smell and a brewing smell. This seasoning is prepared by simultaneously conducting the cultivation of a koji mold (Aspergillus) and the hydrolysis of proteins in the medium by the enzymes contained in the culture of the koji mold in the absence of salt or in the presence of a small amount of salt.
However, the problem has remained that when the hydrolyzate produced by these conventional processes is stored, coloring occurs within a relatively short period of time, and it is browned rapidly, with the result that the commercial value thereof is notably reduced.
Any known processes for producing amino acids by enzymatically hydrolyzing a protein material containing a solid protein have involved problems that microorganisms other than those as the enzyme source in the hydrolysis step, so-called contaminants are grown, reducing the quality of hydrolyzates and decreasing the yield of amino acids. In order to solve the problems, the presence of bacteriostatic materials such as alcohol, sodium chloride and ethyl acetate has been employed in the hydrolysis step in the conventional processes. In these processes, an additional step of separating and removing the bacteriostatic materials after the completion of the hydrolysis step is required. Especially when the presence of sodium chloride is employed as a bacteriostatic means, it has been quite difficult to remove sodium chloride to less than an appropriate concentration without reducing the quality of the resulting hydrolyzate. Further, it has been almost impossible to avoid occurrence of a so-called brewing smell or soy sauce smell in the product obtained through the hydrolysis step in the presence of the bacteriostatic materials, which results in extremely limiting the use range of the hydrolyzed protein obtained.
In addition, in the conventional processes as well, an approach has naturally been attempted in which contaminants incorporated or contained in a protein material containing a solid protein or a microbial culture as the enzyme source are removed or killed, and the hydrolysis step is then conducted. The process in which the starting material is subjected to the proteolysis reaction after the sterilization is said to be relatively easy on a laboratory scale. However, in the industrial mass-production, it involves quite serious problems such as the control of contaminants in the sterilization step and the proteolysis step.